Field of the Invention
The present invention relates to a reclining device having a first member (a ratchet or a seat back side member) having a closed-bottomed cylindrical shape and having internal teeth formed on an inner circumferential surface of the first member along a circumferential direction, a pawl having external teeth capable of meshing with the internal teeth, a second member (a base plate or a seat cushion side member) provided on an opening side of the first member coaxially with and turnably in relation to the first member and having a pawl guide for guiding movement of the pawl between a lock position to prevent the relative turning between the first member and the second member through engagement of the external teeth with the internal teeth, and an unlock position to allow the relative turning through disengagement of the external teeth from the internal teeth, and a cam provided between the first member and the second member coaxially with and turnably in relation to the first member and the second member and urging the pawl to a direction of engaging the external teeth with the internal teeth according to an angle of turn.
Description of the Related Art
For example, JP-2012-200483-A discloses a reclining device which includes a ratchet having a closed-bottomed cylindrical shape and having internal teeth formed on an inner circumferential surface thereof along a circumferential direction, a pawl having external teeth capable of meshing with the internal teeth, a base plate provided on an opening side of the ratchet coaxially with and turnably in relation to the ratchet and having a pawl guide for guiding movement of the pawl between a lock position to prevent the relative turning between the ratchet and the base plate through engagement of the external teeth with the internal teeth, and an unlock position to allow the relative turning through disengagement of the external teeth from the internal teeth, and a cam provided between the ratchet and the base plate coaxially with and turnably in relation to the ratchet and the base plate and urging the pawl to a direction of engaging the external teeth with the internal teeth according to an angle of turn.
In the conventional reclining device, when a strong impact force is applied from the seat back side, the impact force is transmitted, as a force attempting to turn the ratchet (which is a seat back side member), from the internal teeth of the ratchet to the external teeth of the pawl (which is a seat cushion side member). Since the internal teeth of the ratchet and the external teeth of the pawl are formed such that a tooth flank for meshing and mutual contact is tapered in such a manner as to narrow the tooth toward the tooth crest, the tooth flanks of the internal teeth of the ratchet impose force on the corresponding tooth flanks of the external teeth of the pawl in such a direction as to move the pawl upward.
As a result, the external teeth of the pawl are lifted and disengaged from the internal teeth of the ratchet, whereby the pawl attempts to move toward the cam (radially inward). That is, since the contact area between the internal teeth of the ratchet and the external teeth of the pawl reduces, lock strength deteriorates.